1. Field of the Invention
This invention generally relates to a method of forming a turbine shell of a torque converter by press working or stamping a sheet material. More specifically, the present invention relates to a method of forming a radially inner corner portion of the turbine shell by bending the sheet material.
2. Background Information
Torque converters usually include a fluid coupling mechanism for transmitting torque between a crankshaft of an engine and an input shaft of an automatic transmission. A torque converter has three types of runners or vane wheel members (e.g., impeller, turbine and stator) which work together for transmitting the torque from the crankshaft of the engine to the input shaft of the transmission by the movement of an internal hydraulic oil or fluid. The impeller is fixedly coupled to the front cover that receives the input torque from the crankshaft of the engine. The hydraulic chamber formed by the impeller shell and the front cover is filled with hydraulic oil. The turbine is disposed opposite the front cover in the hydraulic chamber. The turbine is fixedly coupled to the transmission. When the impeller rotates, the hydraulic oil flows from the impeller to the turbine, and the turbine rotates. As a result, the torque is transmitted from the impeller to the turbine, which in turn transmits the torque to rotate the main drive shaft of the transmission.
In recent years, to improve fuel efficiency, some torque converters have included lock-up devices that, upon reaching predetermined operating conditions, lock-up the torque converters so that power from the crankshaft of an engine is directly transmitted to the automatic transmission. Thus, lock-up devices bypass the fluid coupling device. Upon engagement, lock-up devices often cause a shudder, or vibration. Further, while engaged, the lock-up device is subjected to vibrations caused by sudden acceleration, or deceleration, or other vibrations including circumstances associated with internal combustion engines. Consequently, torsional vibration dampening apparatuses are typically employed in lock-up mechanisms to dampen vibration.
The lock-up clutch is disposed in the space between the front cover and the turbine. As mentioned above, the lock-up clutch is a mechanism to directly transmit the torque between the crankshaft of the engine and the drive shaft of the transmission by mechanically coupling the front cover and the turbine. The lock-up clutch includes primarily a piston and an elastic coupling mechanism to connect the piston to the members on the power output side of the turbine. The piston is disposed to divide the space between the front cover and the turbine into a first hydraulic chamber on the front cover side and a second hydraulic chamber on the turbine side. As a result, the piston can move close to and away from the front cover by the pressure difference between the first hydraulic chamber and the second hydraulic chamber. A friction joining member covered by friction facing is typically formed on the outer periphery of the front cover on the axial surface facing the piston. When the hydraulic oil in the first hydraulic chamber is drained and the hydraulic pressure in the second hydraulic chamber increases in pressure, the piston moves toward the front cover side. This movement of the piston causes the friction facing of the piston to strongly press against the friction surface of the front cover.
The elastic coupling mechanism functions as a torsional vibration dampening mechanism to dampen vibration in the lock-up clutch. The elastic coupling mechanism includes, for example, a drive member fixedly coupled to the piston, a driven member fixedly coupled to the turbine side, and an elastic member, such as one or more coil springs, disposed in between the drive member and the driven member to enable torque transmission.
When the lock-up clutch is engaged, the hydraulic oil in the first hydraulic chamber is drained from its inner circumferential side and the hydraulic oil is supplied to the second hydraulic chamber. As a result, the hydraulic pressure in the second hydraulic chamber becomes greater than the hydraulic pressure in the first hydraulic chamber. This pressure differential between the first and second hydraulic chambers causes the piston to move toward the front cover.
The turbine shell is a component of the torque converter, which is typically formed by press working or stamping a sheet metal material. The turbine shell generally has a plate thickness ranging from approximately 1.4 mm to approximately 1.6 mm. The turbine shell is typically provided at its radially inner portion with a curved inner corner portion having a radius of approximately 2 mm or less.
When the turbine shell is formed with a curved inner corner portion by using prior art methods of press working or stamping, an internal tensile stress occurs in a direction substantially perpendicular to the direction of the thickness of the plate so that the radially inner curved portion expands. This bending operation results in reduction of the thickness of the radially inner corner portion of the turbine shell. More specifically, the thickness of the curved inner corner portion is reduced by approximately 15-25% from that of the original thickness of the material.
The turbine shell is subjected to an inner pressure of the working fluid of the torque converter. Accordingly, even a local reduction in thickness of the turbine shell may cause expansive deformation, and therefore may cause damages and/or disengagement of parts. In the turbine shells of the prior art, thicker materials have been used in order to avoid the disadvantages caused by the reduction in strength and rigidity due to reduction in thickness. However, the use of a thick material increases the manufacturing cost as well as the weight of the torque converter.
It is necessary to provide a predetermined space between the radially inner corner portion of the turbine shell and the stator for forming a passage of working fluid. In many cases, the radius of the radially inner corner portion of the turbine shell is generally restricted to approximately 2 mm or less in order to ensure a high accuracy in size of this space and to improve the efficiency of flow of the working fluid.
In view of the above, there exists a need for a method of forming a turbine shell with a corner portion which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.